Olefinic unsaturated monomers such as ethylene can often be polymerized in the presence of a catalyst composition, which has essentially two components: a compound of a transition metal belonging to one of groups 4 to 6 of the Periodic Table of Elements (Hubbard, IUPAC 1990) which is often called a procatalyst, and a compound of a metal belonging to any of groups 1 to 3 of said Table which is often called a cocatalyst. This kind of Ziegler-Natta catalyst composition has been further developed by depositing the procatalyst on a more or less inert and particulate support and by adding to the catalyst composition in the stages of its preparation several additives, among others electron donating compounds. These compounds have improved the polymerization activity of the catalyst, the operating life and other properties of the catalyst composition and first of all properties of the polymers which are obtained by means of the catalyst composition.
When ethylene polymers are produced, the polymer molecules formed are not similar by molecular weight, but a mixture having a narrow or broad molecular weight distribution is developed. The broadness of the molecular weight distribution may be described by utilization of the ratio of two different averages, namely the weight average molecular Mw and the number average molecular weight Mn, where a high value of Mw/Mn indicates a broad molecular distribution. For controlling the molecular weight a so called chain transfer agent can be added to the polymerization reaction mixture. In order to obtain polymer products having different molecular weights, different amounts of the chain transfer agent for controlling the molecular weight must be fed into the polymerization reaction mixture. The most usual and preferable chain transfer agent is hydrogen, because when using it no foreign atoms or atom groups are left in the growing molecule, that would cause inconveniencies for the polymerization process or disadvantageous properties of the polymer produced.
How well the molecular weight of the produced polymer varies as function of the hydrogen amount, i.e. how much the so called hydrogen sensibility changes, greatly depends on the catalyst composition. Generally the problem is, that in polyethylene production the polymerization activity decreases to quite an extent the more hydrogen is present.
This absence of catalyst activity balance is a common drawback for all prior art catalysts today. The imbalance shows up when, using prior art catalysts, a drastic drop in the productivity of the catalysts occurs when going from polymerization conditions giving high molecular weight polymers (low melt flow rate) to polymerization conditions giving low molecular weight polymers (high melt flow rate). Even if such a commercial catalyst can have a quite good productivity at a polymer melt flow rate (MER, defined according to standard ISO 1133) of 1, there is often only 10% left of the productivity when producing a MFR of 500. Thus it is desirable to provide a catalyst system having a high activity which is independent of the molar mass of the polymer under formation.
The activity balance discussed above is important in production of bimodal polyethylene. There, a low molecular weight component is produced in one stage at a high hydrogen concentration and a high molecular weight component is produced in another stage at a low hydrogen concentration. Since no fresh catalyst is added between these polymerization stages, the catalyst employed in production of bimodal polyethylene must be able to produce the different molecular weights with a high productivity.
EP-A-32307 discloses a procatalyst that has been prepared by treating an inorganic support like silica with a chlorination agent like ethyl aluminium dichloride which support is then contacted with a magnesium alkyl compound like butyl ethyl magnesium, and with titanium tetrachloride (see claim 1, example 1, table 1).
WO-A-96105236 discloses a catalyst component comprising (i) a particulate support where the majority of particles is in the form of an agglomerate of subparticles and (ii) a magnesium halide. The publication discusses the preparation of the support material. It also describes catalyst preparation and polymerization examples. The catalyst is prepared by adding titanium tetrachloride and DEAC on the agglomerated carrier containing magnesium chloride. The polymerization examples show that a higher bulk density and a higher MFR (better hydrogen response) as well as a lower FRR (narrower molecular weight distribution) is obtaines by the catalyst prepared according to the disclosure. The publication does no refer to the homogeneity of the material.
EP-A-688 794 discloses a process for the preparation of a high activity procatalyst, wherein an inorganic support is reacted with an alkyl metal chloride, the first reaction product is reacted with a compound containing hydrocarbyl and hydro-carbyl oxide linked to magnesium, and the obtained second reaction product is contacted with a titanium chloride compound. The obtained procatalyst has good activity both at high and low MFR polymerization conditions, but it has the draw-back of giving an inhomogeneous ethylene polymer product, resulting in gels and white spots in the polymer material. These inhomogenities have detrimental effect on the appearance and mechanical properties of polyethylene film.